Scanning electron microscope

ABSTRACT

A scanning electron microscope for digitally processing an image signal to secure the largest focal depth and the best resolution in accordance with the magnification for observation is disclosed. The angle of aperture of an optical system having a plurality of convergence lenses is changed by changing the convergence lenses and the hole diameter of a diaphragm. The angle α of aperture of the electron beam is changed in accordance with the visual field range corresponding to a single pixel, i.e. what is called the pixel size.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.12/250,763, filed on Oct. 14, 2008, which is a Divisional of U.S.application Ser. No. 11/397,812, filed Apr. 5, 2006, now U.S. Pat. No.7,442,929, claiming priority of Japanese Application No. 2005-110082,filed Apr. 6, 2005, the entire contents of each of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to a scanning electron microscope for producing atwo-dimensional image by scanning a sample with a converged electronbeam and digitally processing the secondary particle signal generated.

In observing a sample having a large surface roughness, a scanningelectron image is blurred and a clear image cannot be obtained in thecase where the focal depth of the electron beam is smaller than theroughness of the sample. In this case, a clear image can be obtained byreducing the angle of aperture of the electron beam and increasing thefocal depth. A smaller angle of aperture, however, would increase thediameter of the electron beam due to the diffraction effect of adiaphragm and deteriorate the image resolution. Thus, the focal depthand the diameter of the electron beam are in the tradeoff relation toeach other. On the assumption that the resolution of human eyes islimited and therefore the diameter of the electron beam should not bemore than the resolution of human eyes against the scanning electronimage, JP-A-1-236563 proposes an electron microscope in which thediameter of the electron beam is set to at least the minimum angle ofaperture of the electron beam not more than the resolution of human eyesin accordance with the magnification change of the scanning electronimage thereby to attain the maximum focal depth against the setmagnification for observation.

In the scanning electron microscope recently used, the secondaryparticle signal is exclusively subjected to analog-to-digital conversionand projected on a display or the like. In the process, the scanningelectron image is configured of pixels having a finite display area. Theresolution of the scanning electron image, therefore, is not more thanthe visual field area corresponding to one pixel, i.e. what is called apixel size. The resolution of the scanning electron image digitallyprocessed in this way is described in “Nuclear Instruments and Methodsin Physics Research A519 280”. In this reference, the resolution of thedigital scanning electron image is evaluated using the informationpassing capacity (IPC) method. As long as the pixel size is sufficientlysmall as compared with the diameter of the electron beam, the resolutionis substantially equal to the diameter of the electron beam, whereas theresolution is deteriorated with the increase in pixel size. Theresolution of the scanning electron image for the image magnification atwhich the diameter of the electron beam is sufficiently smaller than thepixel size is at most 1.7 Lp, where Lp is the pixel size.

SUMMARY OF THE INVENTION

The pixel size is determined by the visual field area and the number ofpixels. The number of pixels is defined as the number of pixels perscreen in digital conversion. The number of pixels is required to bechanged in accordance with the purpose of observation. In the case wherethe number of pixels is increased to improve the image resolution, thesampling time in analog-to-digital conversion is increased, resulting inan increased image pickup time. For this reason, the number of pixelssufficient to secure the required image resolution is set in accordancewith the size of the sample to be observed and the magnification forobservation. In this way, the digital information for resolution ischanged in accordance with the pixel size to maintain resolution of thescanning electron image.

In JP-A-1-236563, the diameter of the electron beam is calculated fromthe magnification for observation and the image blur amount allowablefor observation. The digital processing according to this method,however, fails to take the resolution of the scanning electron image asa digital image into consideration and cannot keep up with the change inthe number of pixels.

Also, in the image processing of the scanning electron image, theevaluation of the image resolution corresponding to the pixel size isrequired. As an example of the function of automatic regulation of thefocal point, or what is called the auto focusing function of theelectron beam microscope, the following operation is performed.Specifically, an scanning electron image is picked up while changing thefocal point, and the acquired image is differentiated to calculate thefeature amount indicating the strength of contrast. From this featureamount, the optimum focal point is determined. In order to perform thisauto focusing function accurately, the angle of aperture of the electronbeam is required to be set in such a manner as to attain a sufficientresolution of the scanning electron image. In the process, theresolution is required to be evaluated taking the pixel size intoconsideration in the case where the scanning electron image acquired byauto focusing function is digital information.

The object of this invention is to provide a method of securing thelargest focal depth and the optimum resolution for the scanning electronmicroscope for digitally processing the image signal.

In order to achieve this object, according to this invention, there isprovided a scanning electron microscope comprising a means for changingthe angle of aperture of the optical system including a plurality ofconvergence lenses and a means for storing the conditions of theconvergence lenses and the coil for correcting the visual field, whereinthe optical conditions are switched appropriately by setting the angleof aperture of the electron beam in accordance with the pixel size ofthe scanning electron image using the means described above.

Specifically, according to this invention, there is provided a scanningelectron microscope comprising a sample holder for holding a sample, anelectron beam source, a plurality of convergence lenses for convergingthe electron beam emitted from the electron beam source, an objectivelens for radiating the converged electron beam as a micro spot on thesample, a scanning coil for scanning the electron beam on the sample, adetector for detecting the sample signal generated from the sampleirradiated with the electron beam, an analog-to-digital (A/D) converterfor converting the analog detection signal of the detector to a digitalsignal, a storage unit for storing the digital signal converted by theA/D converter as an image signal, and a display unit for displaying animage associated with the image signal stored in the storage unit,wherein the A/D converter is adapted to switch the number of pixels perscreen by changing the sampling rate, and the angle of aperture of theelectron beam is changed in accordance with the pixel size (visual fieldarea per pixel) determined in accordance with the number of pixels perscreen.

In the scanning electron microscope for digitally processing the imagesignal according to this invention, the largest focal depth and theoptimum resolution can be secured in accordance with the change in pixelsize.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of configuration of a scanningelectron microscope according to this invention.

FIG. 2 is a diagram showing an example of configuration of a means forcontrolling the angle of aperture of the primary electron beam.

FIG. 3 is a schematic diagram showing the electron beam track in Low Magmode.

FIG. 4 is a diagram showing the relation between the angle of apertureof the electron beam and the focal depth.

FIG. 5 is a diagram showing the relation between the pixel size and theangle of aperture.

FIG. 6 is a flowchart of the control flow for setting the angle ofaperture.

FIG. 7 is a schematic diagram showing a window for setting anddisplaying the observation conditions.

DESCRIPTION OF THE INVENTION

Embodiments of the invention are explained below with reference to thedrawings.

FIG. 1 is a diagram showing an example of the configuration of ascanning electron microscope according to this invention. A voltage isapplied between a negative electrode 1 and a first positive electrode 2from a high voltage control power supply 15 controlled by amicroprocessor (CPU) 22, so that a primary electron beam 4 is producedfrom the negative electrode 1 with a predetermined emission current. Anacceleration voltage is applied between the negative electrode 1 and asecond positive electrode 3 from the high voltage control power supply15 controlled by the CPU 22, and therefore the primary electron beam 4emitted from the negative electrode 1 is accelerated and proceeds to alens system in subsequent stages.

The primary electron beam 4 is converged by a first convergence lens 5controlled by a first convergence lens control power supply 16, and theunrequired area is removed by a diaphragm plate 8, after which theprimary electron beam 4 is converged as a micro spot on the sample 11held on a sample stage 23 through a second convergence lens 6 controlledby a second convergence lens control power supply 17 and an objectivelens 7 controlled by an objective lens control power supply 21. Thesample stage 23 is controlled by a stage control unit 24. The objectivelens 7 can assume any of various forms including in-lens type, out-lenstype and snorkel type (semi-in-lens type).

The primary electron beam 4 is two-dimensionally scanned on the sampleby a scanning coil 9. The secondary signal (sample signal) including thesecondary electrons 13 generated from the sample 11 by the radiation ofthe primary electron beam 4 proceeds upward of the objective lens 7, andthen is separated by the difference in energy by an orthogonalelectromagnetic field generating unit 12 for secondary signalseparation, followed by proceeding toward the secondary signal detector14. These secondary signals are subsequently detected by the secondarysignal detector 14.

The signals from the secondary signal detector 14 are stored in a memoryof a computer 25 as a two-dimensional image signal through a signalamplifier 20, an A/D converter 29 and a CPU 22. The A/D converter 29 canchange the number of pixels per screen by switching the sampling rate inaccordance with the image pickup time and the image resolution required.The image information stored in the computer 25 is displayed whenevernecessary on an image display unit 26. The signal of a scanning coil 9is controlled by a scanning coil control power supply 18 in accordancewith the magnification for observation.

In this case, the angle of aperture α₁ is determined by the convergenceconditions of the first convergence lens 5 and the second convergencelens 6 and the hole diameter of the diaphragm plate 8.

Next, FIG. 2 shows an example of configuration of a means forcontrolling the angle of aperture of the primary electron beam 4 in theelectron microscope according to this invention.

The convergence point of the primary electron beam 4 due to the secondconvergence lens 6 controlled by the CPU 22 and the second convergencelens control power supply 17, i.e. what is called the second convergencelens image point can be set at an arbitrary point on the light path ofthe primary electron beam 4. For example, the second convergence lensimage point of the primary electron beam 4 can be set at one ofdifferent points such as Z₁, Z₂ in FIG. 2. In the process, the primaryelectron beam 4 set at a second convergence lens image point by thesecond convergence lens 6 can be converged on the sample by theobjective lens 7 controlled by the CPU 22 and the objective lens controlpower supply 21. In the case where the second convergence lens imagepoint is Z₁ or Z₂, for example, a primary electron beam track 30 or aprimary electron beam track 31 is followed downstream of the secondconvergence lens image point of the primary electron beam 4. By changingthe second convergence lens image points and converging the primaryelectron beam 4 on the sample by the objective lens 7 in this way, thespread angle of the primary electron beam reaching the sample 11 can beset.

Also, a second convergence lens control parameter required to setdifferent second convergence lens image points and an objective lenscontrol parameter for converging the primary electron beam 4 on thesample 11 are stored in advance in an optical condition storage unit 33.These parameters are retrieved from the optical condition storage unit33 to control the CPU 22, the second convergence lens control powersupply 17 and the objective lens control power supply 21. In this way,the angle of aperture of the primary electron beam can be quicklychanged. In this case, the function of the optical condition storageunit 33 can be implemented also by the CPU 22 or the computer 25.

With the movement of the second convergence lens image points, thesample point reached by the primary electron beam 4 may move due to theincoincidence between the magnetic field distribution center of thesecond convergence lens 6 and the objective lens 7 and the light path ofthe primary electron beam 4. In order to correct the amount of thismovement, the primary electron beam 4 is deflected by the visual fieldmoving coil 10 controlled by the CPU 22 and the visual field moving coilcontrol power supply 19, so that the position can be corrected. Thecontrol parameters of the CPU 22 and the visual field moving coilcontrol power supply 19 can be stored in advance in the CPU 22 or thecomputer 25.

In this case, two stages of the convergence lens are assumed and theangle of aperture is changed by controlling only the second convergencelens. A similar effect can be obtained using three or more stages of theconvergence lens or by controlling two or more convergence lenses at thesame time. Also, other configurations with a different position andnumber of the diaphragm plates 8 can be employed with equal effect.

Instead of changing the angle of aperture of the electron beam using theconvergence lenses as in the aforementioned case, a similar effect canbe produced by changing the angle of aperture by switching a pluralityof diaphragm holes of different diameters.

According to this embodiment, the angle of aperture of the objectivelens is changed by changing the second convergence lens object point.The electromagnetic lens, however, may have a slow response rate due tothe inductance of the coil. To solve this problem, the angle of aperturecan be changed more quickly by setting the conditions for electron beamconvergence without any crossover point between the second convergencelens 6 and the objective lens 7. These convergence conditions are calledthe Low Mag mode. FIG. 3 shows the concept of an electron beam track inLow Mag mode. In Low Mag mode, the object point of the secondconvergence lens 6 is so long that a slight change of the current in thesecond convergence lens coil can considerably change the secondconvergence lens object point. Even with an electromagnetic lens havinga low response rate, therefore, the angle of aperture can be quicklychanged.

Next, a method of setting the angle of aperture of the electron beam bypixel size is explained.

The relation between the angle of aperture of the electron beam, theresolution and the focal depth is shown in FIG. 4. In the case whereonly the diameter of the electron beam on the sample is taken intoconsideration, the resolution of the scanning electron image isindicated by dotted line in FIG. 4. In this case, the minimum resolutionis obtained at the angle of aperture α₀. Normally, in order to securethe optimum resolution of the scanning electron image and a large focaldepth at the same time, the area in the left half of the graph whereα<α₀ is used. The relation between the angle of aperture and theresolution is acquired by an electron track simulation or experimentsand stored in a memory.

The resolution of the scanning electron image obtained by digitalconversion of the secondary particle signal is indicated by solid line.In the area from A to B in FIG. 4, the resolution remains unchanged byany change in the angle of aperture. This is due to the fact that theresolution of the image is limited by the pixel size. As described inthe reference “Nuclear Instruments and Methods in Physics Research A519280”, in the case where the diameter of the electron beam issufficiently small as compared with the pixel size, the best resolutionis 1.7 Lp for the pixel size of Lp.

For the pixel size of Lp, therefore, both a large focal depth and a highresolution can be secured at the same time by setting the angle ofaperture to α_(min) as indicated at point A. To set the angle ofaperture to α_(min), the convergence lenses are controlled based in themanner described in the embodiments shown in FIGS. 2 and 3.

An embodiment in which the angle of aperture is set with respect to thepixel size is explained with reference to FIG. 5. The abscissa andordinate of FIG. 5 represent the pixel size and the angle of aperture,respectively. In the example indicated by solid line, the angle ofaperture is reduced with the increase in pixel size in the area wherethe pixel size is larger than the point X₁. This is indicative of thefact that the focal depth increases with the decrease in magnificationin the case where the number of pixels is fixed. In the case where thepixel size is smaller than the point X₁, the angle of aperture ismaintained constant. In other words, in the case where the pixel size issufficiently small as compared with the diameter of the electron beam,the resolution is not dependent on the pixel size, and therefore theangle of aperture is maintained constant to increase the focal depth.

An extremely small angle of aperture may scatter the beam due to theproximity effect caused by the Coulomb effect of electrons. This isespecially conspicuously exhibited in the case where the amount of theelectron beam current is large. As in the area indicated by dashed linewhere the pixel size is larger than the point X₃, therefore, the angleof aperture may be constant for not less than a predetermined number ofpixels. An extremely large angle of aperture, on the other hand, thecurrent density at the center of the electron beam track is increasedand so is the flair on the outer periphery of the electron beam. Theincreased flair deteriorates the S/N of the scanning electron image.Even in the case where the pixel size is not sufficiently small ascompared with the electron beam diameter, therefore, the angle ofaperture may be controlled at a constant value as indicated by the areawhere the pixel size is smaller than the point X₂ in FIG. 5.

FIG. 6 is a flowchart for controlling the angle of aperture at the timeof observation. The magnification for observation is set by the keyboard27 or the mouse 28 shown in FIG. 2 (step 601). The magnification forobservation can alternatively be set automatically by storing a seriesof conditions for observation in the computer 25. Next, the pixel sizeis calculated from the set magnification and the number of pixels (step602). Based on the data on the relation between the angle of apertureand the resolution stored in the computer, the optimum angle of aperturefor the set pixel size is calculated (step 603).

With regard to this angle of aperture, the convergence lens current andthe objective lens current are calculated (step 604), followed bycalculating the coil current for moving the visual field (step 610). Thecurrent for the coils of the convergence lenses, the objective lens andthe coil for visual field movement are determined either from therelation between the angle of aperture stored in the memory and thecurrent for the coils of the convergence lenses, the objective lens andalignment or by interpolation based on the data for a plurality ofconditions. Based on these calculations, the convergence lens powersupply and the objective lens current are set (step 605), and so is thevisual field moving coil current (step 611).

After calculating the magnification of the objective lens 7 (step 607),the scanning coil current is determined to secure the set magnification(step 608). Based on the scanning coil current value thus obtained, thescanning coil current is set (step 609).

The currents for the coils of the convergence lenses and the objectivelens, the alignment coil and the scanning coil may be calculated fromthe formula representing the relation with the angle of apertureobtained by the simulation of an electron track or experiments or byinterpolation based on the data on the conditions for several angles ofaperture.

As described above, according to the method of controlling the angle ofaperture by pixel size, the magnification and the number of pixels perscreen can be individually designated while at the same time displayingthe image resolution and the focal depth on the display unit. FIG. 7 isa diagram showing an example of display on the display unit. This windowis displayed on the image display unit 26 to input and display numericalvalues. The desired values of magnification and the number of pixels areinput in the magnification input column 701 and the pixel number inputcolumn 702, respectively. These input operations are carried out by useof the keyboard 27 or the mouse 28. Upon complete input of numericalvalues in the magnification input column 701 and the pixel number inputcolumn 702, the angle of aperture, resolution and the focal depth arecalculated by the computer 25 or the CPU 22. Based on this angle ofaperture, the electron beam is controlled. Also, the focal depth and theresolution are displayed in the resolution display column 703, the focaldepth display column 704 and the aperture angle display column 705 inFIG. 7.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1-5. (canceled)
 6. A scanning electron microscope that displays an imagegenerated by analog-to-digital conversion of signals obtained byirradiation of a sample with an electron beam, comprising: a displayunit that displays a window to enter a magnification and a number ofpixels of the image; and an arithmetic device for calculating an angleof aperture from the entered value of the magnification and the numberof the pixels of the image; wherein an angle of aperture of the electronbeam is controlled depending on the calculated value of the angle ofaperture.
 7. A scanning electron microscope according to claim 6,wherein said arithmetic device for calculates a resolution and a focaldepth.
 8. A scanning electron microscope according to claim 7, whereinsaid display unit displays one of the angle of aperture, the resolutionand the focal depth.
 9. A scanning electron microscope that displays animage generated by analog-to-digital conversion of signals obtained byirradiation of a sample with an electron beam, comprising: a displayunit that displays a window to enter a magnification and a number ofpixels of the image; and an arithmetic device for calculating aresolution, a focal depth, an angle of aperture from the entered valueof the magnification and the number of the pixels of the image; whereinwhen a second magnification that is different to a first magnificationis entered on the window, the arithmetic device causes the display unitto display one of the angle of aperture, the resolution and the focaldepth calculated from the number of pixels of the image at the secondmagnification.